1. Field of the Invention
The present invention relates to a duplex computer system to be used to control process automations or factory automations. Specifically, the invention relates to duplexed computer systems connected with a common communication line (or backplane bus) and a plurality of I/O units to be connected to and controlled by the computer systems. More specifically, the present invention relates to a duplex computer system of the redundancy standby type, in which one computer is engaged in real work (in a control or running status) whereas the other computer is held in a standby status, prepared to take over control of the process or factory automation if the former computer system experiences trouble. Even more specifically, the invention relates to such a duplex computer system which avoids the use of an independent means connected between the two computers for performing the duplex control, i.e., holding one of the two computers in the control status and the other in the standby status.
2. Description of the Prior Art
In the prior art, duplex computer systems of the standby redundancy type have been adopted as one method for enhancing the reliability of automation control systems. Examples of such duplex computer systems are disclosed in U.S. Pat. Nos. 3,503,048, 3,562,761 and 3,864,670.
FIG. 1 is a block diagram showing the structure of one example of a duplex computer system of the prior art, which is disclosed in U.S. Pat. No. 3,864,670. This system is composed of two computer processor units FC1 and FC2 (field controller 1 and 2), a duplex control unit DXC for monitoring the operations of those computers, and a plurality of input/output units IO1 to IOn connected with the two computers via a bus.
The duplex control unit DXC receives ready signals RDY0 and RDY1, which are obtained mainly from self-diagnosis results (such as memory access checks to detect parity errors or a no-response, arithmetic checks, or I/O response checks), that indicate the operation status of the two computers used to perform the duplex control operations. In this arrangement, a first computer is maintained in a control status while the second is held in a standby status whereby the second computer is switched to control status if the first computer experiences trouble or is removed from the system for maintenance.
In the prior art system of FIG. 1, the computer in control status either executes communications with a main computer terminal (not shown) or outputs control arithmetic results to the various input/output units IOi. The computer in standby status performs none of the control arithmetic operations nor the communications but does the self-diagnosis or database equalizations so that it may standby for smoothly switching to control status.
A disadvantage of this prior art system is that it must be equipped with the independent duplex control unit DXC between the two computers, thus complicating its structure. Still worse, if one of the two computers is used to carry out the control operations, or if one computer is extracted from the system, a setting switch for indicating that condition must be operated.
Another system, which has no independent duplex control unit between the two computers, is disclosed in WO86/00439 (i.e., International Application No. PCT/US85/00936). However, this system is not constructed to prevent trouble experienced by one computer from exerting influence upon the other computer. In addition, it cannot be modified simply to a single computer system, which controls a process or factory automation with only one computer.